Null steering or adaptive noise cancelling is a procedure which has been known for many years and is described, for example, in such typical articles as "Adaptive Antenna Systems," by B. Widrow et al., Proceedings of the IEEE, Vol. 55, No. 12, December 1967, and "Adaptive Noise Cancelling: Principles and Applications," by B. Widrow et al., Proceedings of the IEEE, Vol. 63, No. 12, December 1975. In general, null steering is a technique whereby two or more antenna signals are weighted and summed together to form a composite antenna pattern. The pattern is formed in such a manner as to create antenna pattern nulls in the direction of the jamming signals and lobes in the direction of desired signals. Using null steering techniques, nulls on the order of 50dB can be automatically steered in the direction of a jamming signal.
Using, for example, a four channel null steerer, each antenna signal is split into an in-phase component and a quadrature component with a 90.degree. hybrid circuit or the like. The two signal components are then weighted and summed together along with the signal components from the other antenna weighters, in a final summing circuit. By using a 90.degree. hybrid circuit and weighters, a single phasor (any specific signal on an antenna can be represented by a phasor) on a particular antenna can be shifted to any new phase and amplitude desired. If a jamming signal, or any other undesired signal, is present on two antennas, for example, the null steerer will shift the two signals (phasors) such that they are of equal amplitude and opposite phase. When these two weighted signals are then summed together in the final summing circuit, they will cancel, thereby forming an antenna pattern null in the direction of the jamming signal. The process is similar when the jamming signal is present on all four antennas. The number of independent nulls that can be formed is equal to N-7 where N is the number of antennas
The values of the weighters are automatically adjusted by feeding back the output of the final summing circuit to a correlator or mixer, which mixes the output with each of the signal components from the antenna, which is nonweighted, thereby creating a correlation voltage. This correlation voltage is integrated and used to drive the specific weighter for that antenna component. The weighters are always driven in such a manner as to minimize the feedback signal. When the feedback signal is completely eliminated, corresponding to forming a complete null, the output of the correlator is zero and the system has fully adapted. A null steerer implemented in this manner will null out all signals as long as the number of signals is equal to or less than N-1.
To prevent nulling of desired signals, a reference signal must be used. Any prior art null steering systems which utilize a reference, simply insert an estimate of the desired signal. This reference, or estimated signal, is then used to subtract off the desired signal present at the output of the final summing circuit, thereby, preventing it from being fed back to the correlators. If the estimated signal differs from the desired signal in phase or content, a null will also be formed in the direction of the desired signal and the desired signal will be lost. Thus, it is essential that the reference signal be extremely accurate.